The present invention relates generally to a semiconductor device and a process for producing the same, more particularly to a technique of forming multilayered wiring.
Recently, multilayer wiring structures employed in highly integrated semiconductor devices are required to have reduced resistance in inter-wiring contacts (via contacts) and improved wiring reliability.
FIGS. 11 to 13 show, in cross-sectional views, a process for producing a conventional two-layer wiring, which will be described below, step-by-step.
In Step A (see FIG. 11), a silicon oxide film 52 is deposited as an insulating film to an appropriate thickness on the surface of a single crystal silicon substrate 51 by means of CVD (chemical vapor deposition) method. Next, a titanium (Ti) thin film 53, a titanium nitride (TiN) thin film 54, an aluminum (Al) alloy thin film 55 and a titanium nitride (TiN) thin film 56 are deposited successively on the surface of the silicon oxide film 52 by means of sputtering to form a first wiring layer 71.
Subsequently, the thus formed first wiring layer 71 is subjected to patterning employing the conventional photolithographic technique, followed by dry etching to form a wiring pattern of the first wiring layer. It should be noted here that the aluminum alloy thin film 55 contains, in addition to pure aluminum, other metals or high-melting metals (e.g., Al--Si (1%) -Cu (0.5%), Al--Cu and Al--Mg).
Use of such aluminum alloys instead of pure aluminum can prevent electromigration and stress migration from occurring. Electromigration refers to migration of aluminum atoms due to the electron current, while stress migration refers to shifting of the sites where stress is induced by heat, and both phenomena can cause disconnection.
Further, the titanium thin film 53 and the titanium nitride thin film 54 formed under the aluminum alloy thin film 55 are to prevent adhesion at contact sections (not shown) between the aluminum alloy thin film 55 and the substrate 51 from being destroyed by the reaction between Al and Si. If these films 53 and 54 are not present, aluminum in the aluminum thin film 55 reacts with the silicon substrate 51, when heat treatment is carried out after formation of the first wiring layer. Thus, while Al and Si form an eutectoid, the Si is supplied from the silicon substrate 51, so that adhesion at each interface is destroyed. Accordingly, the titanium thin film 53 and the titanium nitride thin film 54 are formed under the aluminum alloy thin film 55 to prevent a reaction at each interface from occurring.
Further, the reason why the titanium thin film 53 is formed under the titanium nitride thin film 54 is that the contact resistance is increased if the titanium nitride film 54 is used alone. As described above, the titanium nitride thin film 54 and the titanium thin film 53 serve as barrier metals. Further, the titanium nitride thin film 56 formed on the aluminum alloy thin film 55 is to prevent the aluminum alloy thin film 55, when subjected to photolithographic light exposure, from reflecting light. In other words, the titanium nitride thin film 56 serves as a reflection preventive film (cap metal).
In Step B (see FIG. 12), a silicon oxide film 57 is deposited as a layer insulating film by means of CVD to an appropriate thickness on the surface of the titanium nitride thin film 56 of the first wiring layer, and patterning of contact holes is carried out by employing a conventional photolithographic technique, followed by formation of contact holes 58 by means of dry etching.
In Step C (see FIG. 13), etching scum in the contact holes 58 and the oxide film present on the surface of the titanium nitride thin film 56 of the first wiring layer 71 in each contact hole 58 is removed by means of sputter etching employing an inert gas (e.g., argon).
Next, a titanium nitride thin film 59, an aluminum alloy thin film 60 and a titanium nitride thin film 61 are deposited successively onto the surface of the silicon oxide film 57 and in the contact holes 58 to form a second wiring layer 72.
Subsequently, the second wiring layer 72 is subjected to patterning employing a conventional photolithographic technique followed by dry etching to form a wiring pattern of the second wiring layer 72 to complete the process of producing the two-layer wiring. The aluminum alloy thin film 60 is of the same material as the aluminum alloy thin film 55.
The titanium nitride thin film 61 formed on the aluminum alloy thin film 60 serves as a cap metal like the titanium nitride thin film 56. Further, the titanium nitride thin film 59 formed under the aluminum alloy thin film 60 is to control growth of hillocks caused by heat treatment such as sintering and alloying. More specifically, since growth of hillocks induces short-circuiting in the wirings, the titanium nitride thin film 54 is formed under the aluminum alloy thin film 60 to control growth of hillocks.
However, in the prior art exemplified above, while an increase in the contact resistance is controlled, for example, by forming the titanium thin film 53 under the titanium nitride thin film 54 in Step A, only the titanium nitride thin films 56 and 59 are present at contact portions between the first wiring layer and the second wiring layer.
Recently, semiconductor devices have become more and more integrated, so that contact holes 58 are required to have smallest possible diameters, and it is essential to prevent contact resistance between the first wiring layer and the second wiring layer from increasing. Under such circumstances, Japanese Unexamined Patent Publication No. 7-142580 describes a laminate structure. In this publication, titanium nitride thin film/titanium thin film is employed at contact portions between the first wiring layer and the second wiring layer, and while the function as cap metals is retained, contact resistance is lowered and electromigration resistance is improved.
Although the prior art semiconductor in the above Japanese publication has excellent contact resistance and electromigration resistance, it is slightly inferior, due to the additional need for the titanium thin film on semiconductors having only a titanium nitride thin film in serving the demand for finer or highly integrated wirings.